Adds help for installing prerequisites on RHEL

Certain prerequisites are not found by yum by default on RHEL. The yum
command specified in the RHEL instructions for installing prerequisites
fails to install python-pip and git-review. Adding a sentence directing
readers to use the EPEL software repository which has the missing
packages.

Closes-Bug: 1392355
Change-Id: Ibcbe9ff9e5d8eeb9f2511915d67dc53ac1dc1b63